Is there a Data Annotation that prevents data duplication in the Database?

Question

How do I prevent data from being duplicated in the database using Data Annotation or via another validation?

Answer 1

public class User
{       
   [Index(IsUnique=true)]
   public string UserName{get;set;}
}

Answer 2

One way to do this is with RemoteValidation, works like this: you should create a ActionResult return a JsonResult with true or false indicating whether the validation has passed. This validation will be done by Ajax if using Unobtrusive Validation.

This is not a type of validation that will create an index in the database, which would make an exception when trying to save duplicate data. The index guarantees you integrity, for sure, but this solution is more user-friendly and it is not necessary to treat the exception flow. Use both solutions if applicable.

In the model:

// primeiro parametro é nome da ação, o segundo do controller
[Remote("ValidarEmailUnico", "Usuario", ErrorMessage = "E-mail já cadastrado")]
public string Email{ get; set; }

User controller:

 public ActionResult ValidarEmailUnico(string Email)
 {
    var emailDisponivel = contexto.Usuarios.Where(u => u.Email == Email).Count() == 0;
    return Json(emailDisponivel, JsonRequestBehavior.AllowGet);
 }

More here: http://www.macoratti.net/15/02/aspn_vremt1.htm

Answer 3

There are some forms, the one that is probably the most suitable for you would be to use the attribute Index to create an index in the database and indicate that it must have a unique key.

public class SuaClasse {       
   [Index("NomeDoIndice", IsUnique = true)]
   public string ColunaUnica { get; set; }
}

I put in the Github for future reference.

It is possible to mount the index with multiple columns.

Don’t forget to add the namespace System.ComponentModel.DataAnnotations.Schema to use this attribute.

Documentation.